Fork lift trucks provide a useful and efficient means of transporting heavy or cumbersome cargo throughout a work-site, such as a construction work site. Fork lift trucks also provide an efficient and graceful means for stowing freight in, say, a warehouse.
Historically, fork lift trucks have usually been quite heavy so as to counterbalance the load carried by the forks. Consequentially, fork lift trucks were difficult to transport on trucks or be propelled over unpaved or soft ground.
U.S. Pat. No. 4,365,921 issued to Brouwer et al. on Dec. 28, 1982, provides an example of a lightweight fork lift truck. The fork-lift tower is mounted on a carriage which rides on a rack within the frame of the fork-lift truck via a drive shaft with gears at its ends which engage the teeth of the rack. The shaft is propelled by means of a drive chain, linked to a sprocket coupled to one side of the shaft and driven by a hydraulic motor attached to the carriage. The racks, gears, sprocket and drive chain are enclosed within the side frame members, which are hollow, to protect them from the environment.
This design eliminates the need to counter-weight the load while transporting it by providing for the fork-tower to be retracted to a point behind the front wheels, providing a reduced moment arm between the load and the fork lift truck's center-of-gravity which prevents forward tilt of the truck while transporting heavy loads.
However, the fork lift truck disclosed in this patent has various disadvantages. For example, the weight of the hydraulic motor on the carriage provides an undesirable increase of the moment force when the carriage is fully extended and holding a load. Also, the complex design of the carriage movement means is detrimental to easy operation and maintenance; by applying the torque to one end of the drive shaft and allowing the other end merely to follow, the drive shaft is capable of skewing and binding while operating, especially when grime or other contamination is present and/or wear of the two side's gears and racks are uneven or when the load on the forks is placed to one side. In addition, maintenance problems inherent with the use of drive chains are well-known and they should be avoided when possible. Further, enclosing the racks, gears, sprocket and drive chain, although necessary to protect these parts from the environment, promotes the difficulty of maintaining these high-maintenance parts. Still further, the fork lift truck disclosed in the Brouwer et al. patent lacks stability under certain operating conditions due to the design of its outrigger system.
There is a need for a lightweight fork lift truck that can stably lift and carry heavy loads, is compact and easily transportable while, at the same time, provides simple operation and maintenance.